It has long been well-known that lead contamination on the interior surface of lacquered and unlacquered can bodies soldered with lead bearing solder is detrimental, particularly where the cans are used for packaging food, beverages, cosmetics and drugs consumed by humans, especially infants. As a result, government health authorities are concerned that this lead which adds to the total lead consumed from all sources, should be below maximum limits for the health and safety of consumers.
One of the most difficult problems in the past has been the control of the spattering of the solder during the soldering operation. Such spattering of the solder results in a relatively large amount of particulate lead being deposited on the inside of the can body in the area adjacent to the seam, especially near the overlapping ends and at the seam laps. The fluxes which have heretofore been used have been responsible for such spattering because when they came in contact with molten solder, at the high temperatures employed during soldering, usually in the vicinity of 350.degree. C., various volatile materials such as water, lower boiling acids and decomposition products were explosively evolved causing the lead matter to spatter, especially as the available exits for the exploding gases are the narrow crevices of the seam and the capillary gaps of the laps.
Conventionally, previous fluxes comprise such substances as zinc ammonium chloride, carboxylic acids, amines, alcohols, water by itself and water, which can be produced as a reaction product between acids and metal oxides during soldering or be present as an impurity. In addition, many carboxylic acids used in previous flux compositions are subject to dehydration or decarboxylation when strongly or rapidly heated, forming gaseous decomposition products such as water, oxides of carbon and low molecular weight olefins together with various oxygenated hydrocarbons such as ketones. In addition, nitrogeneous bases react with derivates of carboxyl groups forming amides and similar substances with the evolution of water and other gases, provided they are sufficiently involatile to remain during heating. Alcohols such as isopropanol, traditionally used as a solvent, assist in retention of water in previous flux compositions as an impurity, sometimes until the critical spattering temperature is reached during soldering, by hydrogen bonding and other coordinations. All of the above substances can be formed or volatilized so rapidly that their evolution is explosive and thus they spatter the particulate lead, lead reaction products of soldering and other matter present during soldering at the interface of the molten solder and previous fluxes. Examples of such fluxes would be a solution in isopropanol of the monoethanolamine, salt of sebacic acid and an isopropanol solution of monoethanolamine hydrochloride with either zinc chloride or a rosin or other carboxylic acid.